I did a replay of the WOL campaign and have come up with a theory. Ariel has been infested the whole time and killing her is the canon ending. The first mission has a two-fold purpose to get all colonists to retreat to a single point for infestation and to get Raynor involved. Meinhoff infested Ariel planned for raynor to lose there so she can get ahold of his forces and his ship, as she is lacking firepower to infest the core worlds. The last mission im thinking is Kerrigan ordering her to fight the protoss seeing as other plan failed, or simply a place to retreat to rebuild.
I’ve had this thought for awhile but havent been able to find info until I replayed and listened to every conversation mid-mission. Ariel is always denying any threat coming from the colonists. Tosh reads ill-intent in raynor helping her after several missions. It was said that all refugees were concentrated on meinhoff before going to the core worlds and if raynor failed there an infested horde would have invaded. It is also stated is that it was weird that meinhoff was attacked so late compared to the rest of the planets like it was planned by someone.
Its that or the cliche infests oneself to test if a vaccine/cure would work but that is far less interesting.